Adaptive optics systems have been devised to improve image resolution by correcting for distortions induced in light wavefronts by atmospheric disturbances and the imperfections of the receiving optical systems. These adaptive optics systems are either outgoing wave modulated systems or return wavefront measurement systems.
Essential elements of such adaptive optics systems is the wavefront sensor which senses wavefront distortion and the wavefront compensator which corrects the wavefront in response to the sensor.
A phase sensitive type of wavefront sensor is the so-called shearing interferometer described by Hardy et al. in "Real-time Atmospheric Compensation", J. Opt. Soc. Amer. 67, 360,1977. In this type of system, the input wavefront is interfered with by a laterally displaced, or sheared, replica of itself. The shear is provided by a set of rotating transmissive gratings, using the interference between the zero (unsheared) and the .+-.1 (sheared) diffraction orders.
The information provided by the shearing interferometer of Hardy et al. is a set of local phase differences between points in the pupil plane separated by the shear distance. By measuring these phase differences in two dimensions using a set of x and y detector arrays, the full pupil can be sampled and the input wavefront approximately calculated.
A drawback of this system is that it uniquely determines phase differences only to within one wave of tilt between points in the pupil plane separated by the shear distance. Larger tilts are detected as having values less than a full wave. This results in a "so-called" two-pi ambiguity. This ambiguity is due to the periodic nature of the amplitude differences produced by phase differences in the two interfering wavefronts. The two-pi ambiguity limits the dynamic range of the shearing interferometer as a wavefront sensor. Phase differences larger than one wave between adjacent subapertures can not be correctly sensed.
Another type of wavefront sensor is the Hartmann-type sensor described in "Integrated Imaging Irradiance (I.sup.3) Sensor, A New Method For Real-Time Wavefront Mensuration", SPIE Vol. 179 Adaptive Optical Components II, (1979) p. 27. In the Hartmann-type sensor, the pupil is divided into subapertures which are each imaged onto an x-y position sensor. The displacement of each spot from its center position yields the average wavefront tilt at its respective subaperture. Such an arrangement allows the measurement of many waves of tilt without ambiguity.
An improvement in the Hartmann sensor is described in U.S. patent application Ser. No. 736,933 filed May 22, 1985 which uses an analog filter array to encode a function of light spot intensity distribution onto the intensity of an image intensified beam of light divided into subapertures and passed through the filter array. This greatly simplifies the centroid calculations needed in prior art Hartmann-type sensors, since pixel weight multiplication is accomplished in an analog fashion by the filter array.
In U.S. Pat. No. 4,472,029 issued to Hardy on Sept. 18, 1984, the Hartmann-type wavefront sensor is integrated with a wavefront compensator. In Hardy's integrated wavefront compensator an input wavefront is divided into a plurality of subaperture images. The subaperture images are transformed by an image intensifier into a corresponding imaged photoelectron charge pattern. The local fields established by the charge pattern are capacitively sensed and a signal proportional to the displacement or changes of the charge locations of the pattern is used to actuate nodes on an active peizoelectric mirror (shown in Feinleib et al. U.S. Pat. No. 3,904,274 issued Sept. 9, 1975) to compensate the input wavefront incident on the mirror surface.
Hardy '029 at column 5 lines 20-35 refers to an earlier Hardy et al. U.S. Pat. No. 3,923,400 and the need to sum the phase differences produced at the nodes. Hardy et al. '400, uses a resistive network for summing while Hardy '029 uses a conductive layer. The conductive layer integrates the phase differences to produce the requisite phase signal to apply to the compensating mirror.
In shearing interferometer and Hartmann-type wavefront sensors, the optical output of the wavefront sensor is processed to yield the tip and tilt of the wavefront at measurement points associated with each subaperture. In both cases, phase differences between adjacent points on the wavefront are measured.